Children whose mothers took antibiotics while they were pregnant were slightly more likely than other kids to develop asthma in a new Danish study.

Children whose mothers took antibiotics while they were pregnant were slightly more likely than other kids to develop asthma in a new Danish study.

"We speculate that mothers' use of antibiotics changes the balance of natural bacteria, which is transmitted to the newborn, and that such unbalanced bacteria in early life impact on the immune maturation in the newborn," said co-author Dr Hans Bisgaard from the University of Copenhagen.

Those effects on the immune system could lead to asthma later on, although it's still not clear how, said Dr Anita Kozyrskyj from the University of Alberta, who also studies the antibiotics-asthma link but wasn't involved in the new study.

Previous research has linked antibiotics taken during infancy to a higher risk of asthma; although some researchers have disputed those findings.

How the study was done

To look for effects starting even earlier in a baby's development, Dr Bisgaard and his colleagues gathered information from a Danish national birth database of more than 30 000 children born between 1997 and 2003 and followed for five years.

They found that about 7 300 of the children, or nearly one quarter, were exposed to antibiotics while their mothers were pregnant. Among them, just over 3% (238 kids) were hospitalised for asthma by age five.

In comparison, about 2.5%, or 581, of some 23 000 kids whose mothers didn't take antibiotics were hospitalised for asthma.

After taking into account other asthma risk factors, Dr Bisgaard's team calculated that the children who had been exposed to antibiotics were 17% more likely to be hospitalised for asthma.

These children were also 18% more likely to have been given a prescription for an asthma medication than kids whose mothers did not take antibiotics when they were pregnant, according to findings published online in The Journal of Pediatrics.

What the study found

Dr Bisgaard said he expected to see a higher risk of asthma "because the mother is a prime source of early bacterial colonisation of the child, and antibiotics may (have) disturbed her normal bacterial flora."

Dr Bisgaard's team also looked at a smaller group of 411 kids who were at higher risk for asthma because their mothers had the condition and found these children were twice as likely as their peers to develop asthma too if their mothers took antibiotics during the third trimester of pregnancy.

Dr Kozyrskyj, who is research chair of the Women and Children's Health Research Institute, said it's also possible that something other than the antibiotics are to blame for the findings in both groups of children - such as the illness that caused the mothers to take antibiotics.

"This study, it doesn't tell us whether it's the antibiotic use or whether it's the infection. That's one thing we can't decipher," she said.

The results don't suggest that women should avoid taking antibiotics to try to reduce their kids' risk of asthma, Dr Kozyrskyj emphasised. Some infections can be quite dangerous to a foetus, she noted.

Dr Bisgaard agreed that's sometimes the case, but added in an email, "We see 1/3 of pregnant women in our region receiving treatments (often for urinary tract infections), which may reflect an uncritical use."

Dr Bisgaard said his group is also studying the types of bacteria in pregnant mothers and newborn children to get a better understanding of their role in asthma.

Dr Kozyrskyj said Dr Bisgaard's study suggests that the development of asthma might start before birth, something researchers hadn't studied very closely.

"We're beginning to appreciate that some of the origins of asthma and changes to the immune system, maybe they start earlier than right after birth. It might be happening in utero," she said.

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